1. Field of the Invention
The present invention relates generally to an audio signal processing apparatus and an audio signal processing method for a multi channel audio reproduction system, more particularly to an apparatus for allowing conversion from a plurality of digital audio signals having different sampling frequencies into analog audio signals.
2. Description of the Related Art
For instance, digital audio signals including information with respect to sound and voice are recorded on a DVD or the like as digital audio signals. The term DVD is to be broadly interpreted to include, but is not limited to, Digital Versatile Disk, Digital Video Disk, and the like, and their equivalents. A DVD reproduction system reads the digital audio signals from the DVD, converts the digital audio signals into the corresponding analog audio signals by using a digital-to-analog converting circuit (DA converter) installed therein, and feeds the analog audio signals to the speakers or the like.
A variety of DVD reproduction systems are developed. Among such DVD reproduction systems, there is a DVD reproduction system which adopts a multi channel audio reproduction method. The multi channel audio reproduction method is a method of delivering audio signals to more than two channels. In such a DVD reproduction system, audio signals are delivered to, for example, six channels, such as the center front, the left front, the right front, the center back, the left back and the right back.
Generally, in such a DVD reproduction system which adopts the multi channel audio reproduction method, all of the audio signals delivered to the respective channels have the same sampling frequency. For example, in case that the sound of movies or motion pictures recorded on the DVD are reproduced by using a DVD reproduction system having six channels, all of the six audio signals delivered to the respective six channels have the same sampling frequency of 48 kHz.
In general, if the sampling frequency of an audio signal is increased, the quality of the sound reproduced by a DVD reproduction system is enhanced. For example, in the DVD reproduction system having six channels, if the sampling frequencies of all the six audio signals are increased from 48 kHz to 96 kHz, the sound quality is enhanced. However, if the sampling frequencies of all the six audio signals are increased, the amount of the audio signals, that is, the amount of data necessary for reproducing the sound is largely increased. In consideration of limitation of the capacity of the DVD and limitation of the data transfer rate with respect to the audio signals, increase in the amount of the audio signals is undesirable.
In case of the DVD reproduction system having six channels, in order to restrict the increase in the amount of the audio signals, it is preferable to increase the sampling frequencies with respect to only some of the six channels, for example, three channels, from 48 kHz to 96 kHz.
Generally, when listening to the music or the sound of the movie, listener's attention is focused on the sound coming from the front as compared with the sound coming from the back. In case of the DVD reproduction system having six channels corresponding to the center front, the left front, the right front, the center back, the left back and the right back, the sampling frequencies of the audio signals to be delivered to the three front channels, i.e., the center front, the left front and the right front, are set at 96 kHz, and the sampling frequencies of the audio signals to be delivered to the three back channels, i.e., the center back, the left back and the right back, are set at 48 kHz. Thus, the increase in the amount of the audio signals can be restrict, while enhancing the overall sound quality of music and movies.
As discussed above, in order to restrict the increase in the amount of the audio signals, it is preferable that the sampling frequencies of the audio signals delivered to the respective channels are set at two kinds of values, for example, 48 kHz and 96 kHz.
However, if the sampling frequencies of the audio signals are set at two kinds of values, two kinds of digital-to-analog converting circuits are required. For example, a digital-to-analog converting circuit to convert the digital audio signal having the sampling frequency of 48 kHz into the analog audio signal and a digital-to-analog converting circuit to convert the digital audio signal having the sampling frequency of 96 kHz into the analog audio signal are required. As a result, the structure of circuits in the DVD reproduction system becomes complex, and manufacturing cost of the DVD reproduction system is increased. For example, the number of signal lines to be used for controlling the respective digital-to-analog converting circuits is increased. Further, in case that multi channel type digital-to-analog converting chips, such as dual channel type digital-to-analog converting chips are used as the digital-to-analog converting circuits, the number of the chips is increased.